Short optical pulse generation has become an increasingly important technology in recent years in many applications including laser-based micromachining, thin-film formation, laser cleaning, medicine and biology. Exciting results have been demonstrated with ultra-short pulses in ablation of a wide variety of materials with a minimum of thermal or shock damage to the surrounding materials. Examples include dielectrics, e.g. oxide ceramics, optical glasses, polymers, etc. Short pulses are also powerful instrumentals for surface patterning and micro-fabrication due to the non-contact character of material processing. In particular, higher spatial resolution can be achieved with short pulses by reducing heat-affected zone and shock-affected zone (cf. e.g. X. Liu, D. Du, and G. Mourou, “Laser ablation and micromachining with ultrashort laser pulses”, IEEE J. Quantum Electron., vol. 33, pp. 1706-1716, 1997)
Two of the most critical aspects of short pulse laser systems are:    1. To avoid non-linear deteriorations of the optical pulses in the system due to non-linear effects in the intra- or extra-cavity laser system    2. To compress the width of the optical pulses inside or outside the laser cavity. This is typically done by dispersive optical elements
Pulse Compression
The pulse width of optical pulses propagating in dispersive medium will be either reduced or increased as they propagate through the dispersive medium.
Dispersion within a laser cavity is typically undesired as it typically will broaden the pulse width. In some situations, however, such dispersion may be highly desirable, as it may increase the obtainable pulse energy before any non-linear distortions are introduced. This dispersion is caused either by the material or waveguide dispersion of the laser medium (or by optical elements such as prisms or gratings) or by a nonlinear optical phenomenon—the Kerr effect—in which the refractive index of a material is dependent on the light intensity. Regardless of the cause of the dispersion in the laser cavity, the pulse broadening caused by this dispersion has to be compensated to allow very short pulse (typically <5 ps) operation of the laser.
Dispersion outside the laser cavity (extra-cavity) can be desirable as it can be used for pulse compression provided that the pulses entering the extra-cavity medium has the appropriate chirp. Typically the chirp of short pulse lasers will require anomalous dispersion for pulse compression.
In most solid-state lasers manufactured today, the necessary intra- or extra-cavity anomalous dispersion is supplied in the form of prism or grating pairs. A prism or grating pair can be used to compensate for the normal dispersion in the cavity. However, this technique is not applicable to compact and robust lasers.
Although waveguide dispersion in prior art laser systems has been used generally to balance the material dispersion at longer wavelengths (>1.3 μm), this is not feasible in traditional fibers at short wavelengths (<1.3 μm). In traditional fibers, anomalous (positive) dispersion at short wavelengths will require very small core sizes (<4 μm), which will lead to very low non-linear thresholds (i.e. increased non-linear effects) in these fibers. If these fibers were used for dispersion compensation or pulse compression in short pulsed lasers, non-linear effects in the fibers would cause the short pulses propagating in these fibers to break up and thereby destroy the short pulse operation of the laser system.